A technique employed in oil and gas well drilling, completion and workover operations involves dropping or pumping a sealing member down the well pipe onto a seat assembly. The sealing or drop member is typically a ball or dart. After the drop member lands on the seat assembly, it forms a seal, allowing the operator to apply increased fluid pressure in the well pipe above the seat assembly. A shear member shears, allowing the seat assembly to move downward to perform a specified function, such as opening or closing ports in the well pipe.
In one use, a seat assembly of this nature may form part of an annular communication valve, which is utilized at some point during the drilling or producing life of an oil or gas well to perform a variety of services. The valve can be used in a system to reduce surge pressure while deploying a liner. Typically, the valve includes a sliding sleeve and a seat assembly deployed in the open position with the sliding sleeve positioned above ports that are open to the annulus. Once the ball has landed on the ball seat assembly, applied pressure shifts the sleeve to cover and seal off the ports.
If running a liner string, cementing may occur after the ball has been released. It is important that the valve be shifted to the fully closed position before the ball is released. Otherwise, an operator might not realize the ports are still open and begin pumping cement down the pipe string. Instead of all of the cement flow out the lower end of the liner string, as required, some of the cement could inadvertently be diverted out the still open valve ports.
In some cases, tools located below the valve will require activation by dropping/pumping another ball or dart to the tool. Therefore, it is desirable for the valve to contain a ball seat assembly that permits passage of another and larger diameter ball once the valve has been shifted by the first ball. The reason for requiring a larger diameter ball is to avoid the first ball inadvertently shifting a lower seat valve after it has shifted and passed through the upper seat. Additionally, the second ball used to land on the lower seat must not be so large in diameter that it has difficulty passing through the upper seat after it has shifted. Breakaway or yieldable ball seat assemblies are known to provide for passage of the ball after shifting. However, if a second ball has to pass through the upper seat after it has broken away, problems can occur. The second ball may have difficulty passing through the upper seat after it has broken away, requiring greater pressure force. Additionally, if the sliding sleeve travel is impeded, the ball can possibly be forced though the yieldable seat assembly without fully closing the sleeve.